In electronic circuit practice it is often necessary to combine two signals of different phase to produce a resultant that has phase information directly related to the inputs. Typically two signals are added to produce an output signal that has a phase intermediate between the phases of the input signals, or the signals are applied to a detector which produces an output related to the difference in phase. When sine waves are employed as signals, conventional linear circuits can readily be employed for such signal processing. However, when the signals are square waves, such processing becomes confused, particularly when limiting is present and ordinary linear circuits cannot be used directly. Square wave signals are of great interest in microelectronic devices because great performance benefits have been realized by the use of digital circuitry.
Typically, where phase information is to be preserved in a square wave system the square wave is passed through a filter, which desirably converts the square wave into its fundamental sine wave equivalent. Then the signal combination is achieved conventionally and the resultant converted back into a square wave by means of amplification and signal limiting. This approach has the great disadvantage of requiring filter components that are difficult to achieve on a microelectronic circuit chip, thereby necessitating externally added components or extreme circuit complexity. Additionally, such filters are of restricted bandwidth. That is, they introduce signal phase shifts that increase with departure from design frequency.